Re-add GUC track_wal_io_timing

This commit is a rework of 2421e9a51d, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads.  Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths.  The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal.  pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled.  The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu

src/backend/utils/activity/pgstat_backend.c

@@ -24,6 +24,7 @@
 
 #include "postgres.h"
 
+#include "access/xlog.h"
 #include "storage/bufmgr.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
@@ -43,7 +44,7 @@ void
 pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context,
 								IOOp io_op, instr_time io_time)
 {
-	Assert(track_io_timing);
+	Assert(track_io_timing || track_wal_io_timing);
 
 	if (!pgstat_tracks_backend_bktype(MyBackendType))
 		return;

src/backend/utils/activity/pgstat_io.c

@@ -83,20 +83,22 @@ pgstat_count_io_op(IOObject io_object, IOContext io_context, IOOp io_op,
 }
 
 /*
- * Initialize the internal timing for an IO operation.
+ * Initialize the internal timing for an IO operation, depending on an
+ * IO timing GUC.
  */
 instr_time
-pgstat_prepare_io_time(void)
+pgstat_prepare_io_time(bool track_io_guc)
 {
 	instr_time	io_start;
 
-	if (track_io_timing)
+	if (track_io_guc)
 		INSTR_TIME_SET_CURRENT(io_start);
 	else
 	{
 		/*
-		 * There is no need to set io_start when an IO timing GUC is disabled,
-		 * still initialize it to zero to avoid compiler warnings.
+		 * There is no need to set io_start when an IO timing GUC is disabled.
+		 * Initialize it to zero to avoid compiler warnings and to let
+		 * pgstat_count_io_op_time() know that timings should be ignored.
 		 */
 		INSTR_TIME_SET_ZERO(io_start);
 	}
@@ -119,7 +121,7 @@ void
 pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 						instr_time start_time, uint32 cnt, uint64 bytes)
 {
-	if (track_io_timing)
+	if (!INSTR_TIME_IS_ZERO(start_time))
 	{
 		instr_time	io_time;
 

src/backend/utils/misc/guc_tables.c

@@ -1501,6 +1501,15 @@ struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+	{
+		{"track_wal_io_timing", PGC_SUSET, STATS_CUMULATIVE,
+			gettext_noop("Collects timing statistics for WAL I/O activity."),
+			NULL
+		},
+		&track_wal_io_timing,
+		false,
+		NULL, NULL, NULL
+	},
 
 	{
 		{"update_process_title", PGC_SUSET, PROCESS_TITLE,

src/backend/utils/misc/postgresql.conf.sample

@@ -641,6 +641,7 @@
 #track_counts = on
 #track_cost_delay_timing = off
 #track_io_timing = off
+#track_wal_io_timing = off
 #track_functions = none			# none, pl, all
 #stats_fetch_consistency = cache	# cache, none, snapshot